Gen-X Cops opens with late-breaking news on TV. The Hong Kong police are celebrating the closing of a major case, and a gangster named Dinosaur (Gordon Lam) needs to flee town in a hurry. In the car, Dinosaur picks on his English-speaking younger brother, Daniel (Daniel Wu): "You became a damn gangster in Canada!"
Dinosaur's car is cut off by Akatora (Toru Nakamura), a black trenchoat-wearing yakuza member. Daniel, in cahoots with Akatora, shoots his brother. Akatora tells Dinosaur a rambling tale about a childhood ghost, then kills Dinosaur, telling him: "From now on, we are family."
Back at police headquarters, Inspector Chan (Eric Tsang) is championing Daniel for the death of his brother. Superintendent To (Moses Chan) mocks Chan, but permits him to work a "special assignment" case, hiring undercover officers to infiltrate Daniel's nightclub.
Chan heads over to the local police academy to recruit. No one seems perfect until three t-shirt clad youngsters enter: Jack (Nicholas Tse), Match (Stephen Fung) and Alien (Sam Lee). They mock the head of the school, and promptly get expelled from the Academy. Chan has found his perfect undercover cops.
Chan catches up with the trio, who are preparing to jump out a plane. The Gen-X gents cut Chan a deal: jump from the plane, and they'll do anything he wants. Anything he wants? My thoughts stray towards champagne, leather restraints and a creaky rocking chair, but what Chan has in mind is far kinkier still: a complete fashion overhaul and lunch at TGI Friday's. Sick bastard.
At the restaurant, Superintendent To runs into Chan and needles him about his undercover team ("If they can solve this case, I'll get down on my knees and serve you a cup of tea"). To doesn't seem to have his heart into the whole mockery business today.
Jack, Match and Alien meet up with computer whiz gal Y2K (Grace Yip) and infiltrate Daniel's club. Daniel enters shortly thereafter, followed by the oddest assortment of henchmen in the New Territories: Yanni, the cowboy hat-wearing Westerner, Tooth (Terence Yin), the Elvis look-alike in shiny black vinyl, and Haze (Jaymee Ong), the beautiful Australian party girl. Haze scopes out out Match, who blithely ignores her.
While Haze is flirting with Match, Daniel meets with his brother's boss, Lok (Francis Ng). Lok demands half a million from Daniel for reward money to find Dinosaur's killer. Since Daniel is the murderer, he doesn't seem too thrilled about paying Lok.
Back at the bar, Haze puts the moves on Match ("Don't I know you? Aren't you from Canada?") Daniel's henchmen notice, and a barfight breaks out. Glass is shattered, kicks are thrown and the Gen-X cops are eventually cornered and trapped. Jack, Match & Alien try one last bluff to get out of the club, to no avail.
Match quickly informs Daniel that Haze was hitting on him, not the reverse. Haze agrees, much to Daniel's surprise. She adds that she was seeing Match in Toronto, and wallops him a good one upside the head. Match finally remembers: "Wong Suk-Mei. I didn't know you were that serious."
Daniel insists that our three boys do him a favor in return for destroying his club: deliver the five million to Lok. Later, in the bathroom with Tooth, Daniel mentions that it'll be the perfect way to get rid of everyone.
At Lok's hideout, Lok introduces himself then offers them gigantic Cuban cigars. Everyone smokes up eagerly. Lok sizes them up, and then takes Jack to the back room. Alien makes a crack about Lok being gay.
Daniel calls Jack on the phone, and tells him to kill Lok. Lok senses something is amiss and holds a gun to Jack's head. Jack smooth-talks his way outand cuts a deal with Lok, pretending to hold him hostage.
Outside, Daniel smugly tells Lok that he's the one that killed his own brother. Lok tells Daniel that he's a complete idiot. Daniel gears up to shoot Lok, then freezes as Tooth cocks his shotgun against his head ("You're damn right"). Tooth double-crosses Daniel for a cool ten million from Lok.
The police are outside, the warring Triad gangs are inside, and Jack, Match & Alien are trying to escape without getting killed. Tooth uses Daniel as a human shield, but is shot and killed. Daniel lobs a couple handy grenades at Lok, and then escapes with Jack. Match & Alien steal a boat and putter them way to safety. A bloodied Lok watches the receding boat.
Daniel and the boys hide out in Shek O. Daniel's waiting for his money and passport from Haze, and our guys have just found out that they're considered armed & dangerous fugitives. Haze eventually arrives, clad in what looks to be a dead yeti pelt. Only Jaymee Ong could pull off a knee-length fur vest and natty scarf combination.
Match approaches Haze and woos her: "I knew you were back in Hong Kong, but I avoided you. How can I look after someone else if I can't look after myself?" This seems like a dumb reason to dump Haze, but she falls for it. It's finally dawning on Daniel that Haze is dumping him for Match. Cutting his losses, he scoops up Jack and drives off. Match dumps Haze, and hops on a motorcycle with Alien, promising to meet up with her later. Haze solemnly tells Match: "If you break this promise, you'll never see me again."
Daniel and Jack approach Akatora, hanging out at an abandoned school in Tai Po. Akatora glamorously smokes and watches a TV news report about reformed yakuza Shimada's upcoming visit to Hong Kong. Nearby, a human-shaped box is rattling ominously. Akatora opens it up and presents a bound and bloody Lok to Daniel. He mentions that Daniel betrayed him, and that Jack's an undercover cop. Lok insults Daniel ("Why is Dinosaur's brother such a pig?"), and Akatora's men kill Daniel. A bemused Lok verbally attacks Akatora and gets tossed into a flaming swimming pool with Jack. Lok nobly sacrifices himself to save Jack.
Jack and Y2K meet up with Inspector Chan, and trail Akatora to Yanni's (the Western drug dealer) place. Akatora traps them up on the roof, and offers them the choice of jumping or being shot down. Inspector Chan quickly provides a distraction, and Match & Alien don skydiving suits, conveniently provided by a sporting goods store in Yanni's building. Everyone dives off the building, but Akatora shoots and kills Inspector Chan.
At the Hong Kong Convention Centre, Akatora prepares to kill his family rival, Shimada. Y2K distracts Shimada, and Jack, Match & Alien go after Akatora. Akatora tells them that he's started the countdown on a bomb to completely blow up the convention center, and that they have only five minutes remaining.
The Gen-X cops attack Akatora and manage to wrestle the detonator away from him. Unfortunately, Match drops and destroys the detonator completely. Akatora solemnly tells them that it's time to die, and everyone (except Akatora) makes a mad scramble for the exits.
Jack, Match & Alien beat the bomb, but are blasted deep into Victoria Harbor, where a local fisherman (Jackie Chan) retrieves them. They sass him and he tells them to be more polite, while the remains of the Hong Kong Convention Centre smolder in the background.
Final scene, Jack, Match and Alien are serving community time cleaning up the trash on the beach. Inspector Chung & To approach, offering them a position back on the police force. They accept, but not before Alien manages to finagle a trip to Hawaii, Match a meeting with Haze, and Jack a promised serving of tea from Inspector To, out of the police department. Y2K can't quite decide what she wants, so those rambunctious lads throw her in the water. "You Can't Stop Me" blares in the background, as the credits roll.